The WaitForMajorityService was introduced in 4.4 (in SERVER-40785) to allow asynchronously waiting for majority write concern. The service holds a queue of opTimes to wait for and has a background thread that periodically waits for the lowest time.

As of SERVER-43135, the ReplicationCoordinator has a futures based API to wait for write concern internally, so it's possible we can refactor the WaitForMajorityService to use that API. This could simplify the service and likely improve its performance, since the current implementation scales poorly with many requests - see SERVER-79785.